Sri Mongkol (SMK) is providing passennger bus service going to east area of Thailand such as:
Si Sa Ket
Their bus also goes to Kanchanaburi.
Their bus terminal in Pattaya is located at Third Road, close to intersection of Third Road and Pattaya Klan (Central) Road. The terminal is easy to reach from Pattaya central area, since you can walk from the intersection.